channeld: use explicit --experimental-upgrade flag, not #ifdef EXPERIMENTAL_FEATURES

And no longer insist on opt_quiesce.

Changelog-EXPERIMENTAL: Config: `--experimental-upgrade-protocol` enables simple channel upgrades.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-listconfigs.7.md

@@ -64,6 +64,7 @@ On success, an object is returned, containing:
 - **experimental-websocket-port** (u16, optional): `experimental-websocket-port` field from config or cmdline, or default
 - **experimental-peer-storage** (boolean, optional): `experimental-peer-storage` field from config or cmdline, or default *(added v23.02)*
 - **experimental-quiesce** (boolean, optional): `experimental-quiesce` field from config or cmdline, or default *(added v23.08)*
+- **experimental-upgrade-protocol** (boolean, optional): `experimental-upgrade-protocol` field from config or cmdline, or default *(added v23.08)*
 - **database-upgrade** (boolean, optional): `database-upgrade` field from config or cmdline
 - **rgb** (hex, optional): `rgb` field from config or cmdline, or default (always 6 characters)
 - **alias** (string, optional): `alias` field from config or cmdline, or default
@@ -225,4 +226,4 @@ RESOURCES
 
 Main web site: <https://github.com/ElementsProject/lightning>
 
-[comment]: # ( SHA256STAMP:62e8c380f4fa7b063c145ad43428c4ce48cb142cab61da72a26c2961375fabab)
+[comment]: # ( SHA256STAMP:4e361c39e45812c4dd1e168c3e4e0960f06511850db7b70475b4b547e92f3bc2)

doc/lightningd-config.5.md

@@ -725,6 +725,14 @@ store as well, based on a protocol similar to [bolt][bolt] #881.
   Specifying this option advertizes `option_quiesce`.  Not very useful
 by itself, except for testing.
 
+* **experimental-upgrade-protocol**
+
+  Specifying this option means we send (and allow receipt of) a simple
+protocol to update channel types.  At the moment, we only support setting
+`option_static_remotekey` to ancient channels.  The peer must also support
+this option.
+
+
 BUGS
 ----
 

doc/schemas/listconfigs.schema.json

@@ -147,6 +147,11 @@
       "added": "v23.08",
       "description": "`experimental-quiesce` field from config or cmdline, or default"
     },
+    "experimental-upgrade-protocol": {
+      "type": "boolean",
+      "added": "v23.08",
+      "description": "`experimental-upgrade-protocol` field from config or cmdline, or default"
+    },
     "database-upgrade": {
       "type": "boolean",
       "description": "`database-upgrade` field from config or cmdline"